[Bug 1862938] Re: Enable late loading of microcode by default

[Steve Beattie]

Hey Dimitri,

Can you expand on what problems/situations where you're actually seeing
late loading be a solution?

The reason I ask is that every communication I've had with Intel has
indicated that late loading is risky and should not be used. The reason
for this is that performing late loading on a running system can result
in race conditions where cpu cores have different values for MSRs/cpu
flags, or even have them disappear momentarily while the microcode is
loading. This can cause a variety of problems for virtual machine
hosts/hypervisors.

Also this statement:

  "For example, from time to time, certain microcode updates are pulled
or get blacklisted from late loading."

isn't really a reason to do late loading.

Finally, why is this being done via tmpfiles.d(5)? If we're really going
to do this, should it not be its own systemd unit, rather than hijacking
something that isn't related?

Thanks.

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Title:
  Enable late loading of microcode by default

Status in intel-microcode package in Ubuntu:
  Fix Committed
Status in intel-microcode source package in Xenial:
  New
Status in intel-microcode source package in Bionic:
  New
Status in intel-microcode source package in Eoan:
  Fix Committed
Status in intel-microcode source package in Focal:
  Fix Committed

Bug description:
  [Impact]

   * An explanation of the effects of the bug on users and

   * justification for backporting the fix to the stable release.

   * In addition, it is helpful, but not required, to include an
     explanation of how the upload fixes this bug.

   * Normally intel microcode is applied "early" for an uncompressed
  prepended initramfs archive. However, on systems booting without an
  initrd, or a missbuilt one, microcode might not get applied. In that
  case, we need to attempt loading microcode late which may give users
  security protection against CPU vulnerabilities which they might
  otherwise be lacking. In an ideal world, everyone would apply their
  bios/OEM updates with microcode updates in a timely fashion and then
  we wouldn't need to update CPU microcode from userspace at all.

  [Test Case]

   * Install updated package
   * Reobot
   * Observe early application of microcode

  $ journalctl -b | grep microcode
  Feb 12 12:02:48 ottawa kernel: microcode: microcode updated early to revision 0xd6, date = 2019-10-03

   * Remove /usr/share/initramfs-tools/hooks/intel_microcode to prevent correct generation of early microcode updates
   * Rebuild initrd with update-initramfs -u
   * Reboot
   * Observe in dmesg that late loading of microcode is performed

  $ journalctl -b | grep microcode
  Feb 12 12:32:54 ottawa kernel: TAA: Vulnerable: Clear CPU buffers attempted, no microcode
  Feb 12 12:32:54 ottawa kernel: MDS: Vulnerable: Clear CPU buffers attempted, no microcode
  Feb 12 12:32:54 ottawa kernel: microcode: sig=0x506e3, pf=0x20, revision=0xc6
  Feb 12 12:32:54 ottawa kernel: microcode: Microcode Update Driver: v2.2.
  Feb 12 12:32:57 ottawa kernel: microcode: updated to revision 0xd6, date = 2019-10-03
  Feb 12 12:32:57 ottawa kernel: x86/CPU: CPU features have changed after loading microcode, but might not take effect.
  Feb 12 12:32:57 ottawa kernel: microcode: Reload completed, microcode revision: 0xd6

  (Note the lack of "early" in above messages)

  [Regression Potential]

   * Application of microcode is a risky operation, especially if the
  cores are busy. Hence we prefer bios updates & early microcode
  updates, and those will remain the place. The late loading of
  microcode is really here for the cases were the previous two update
  strategies have failed. For example, from time to time, certain
  microcode updates are pulled or get blacklisted from late loading.

  [Other Info]
   
   * The majority of our users on bare-metal machines boot correctly with early microcode updates.

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